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DXT-MAX - Small active monitors

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  • #31
    Hi a4eaudio,
    https://sbacoustics.com/product/5x8in-sb15sfcr-00/ shows +/- 11 mm xmech.
    To me honest, I haven't had much time play with these. I have a 2 year old and I mentor a high school robotics team and we are in our build season. I'm no expert, but I would recommend them for a small 2-way build. Being able to use some sort DSP to boost the lower end (to reach the 30's) and take advantage of the excursion is a nice plus. Javad built Project Sledgehammer which uses them in a MTM.
    My Builds - Vigor (Nola Brio Knockoff) - Overnight Sensations w/ Voxel - Speedsters in surplus boxes - Zaph B3N's in bamboo - Classix II in BR-1 cabinets - Dual TPA3116 D2 amp in an old music box - Mariposas

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    • #32
      Originally posted by silverD View Post
      The cheapest miniDSP uses a similar ADAU chip to the DSPB-K which is essentially what I'm currently using but also has opamps on the outputs. The amp powering the tweeter is an Icepower 125asx2 which has a CMoy preamp after the volume pot. I'm looking to utilize a 50asx2SE without preamp or volume in the near future and we'll see if this helps. I have made some changes to the filters, but your suggestion is a good one.
      I’ve used several ADAU1701 DSP units (Sure/Wondom/Dayton, miniDSP, 3e Audio and Tinysine). I’ve found a common cause of the hiss problems is not paying attention to matching gain stages. This is especially true for the ADAU1701 chip because it has a few limitations. Looking at the different areas:
      1. Input stage. The ADAU1701 can handle either 1 or 2 Vrms on the input, depending on the resistors used on the board. The miniDSP 2 X 4 basic is nice since it is jumper selectable. The basic rule is to use the full input range to reduce the SNR. This is particularly true for ADAU1701 since its ADCs are only rated for a THD+N of -83 dB. Nearly any preamp should be able to deliver 1 or 2 Vrms, but some Bluetooth modules and phones have notoriously low outputs.
      2. Output stage. The ADAU1701 DACs can output 0.9 Vrms before clipping. The miniDSP 2 X 4 basic may have op-amps on the output, but is still limited to 0.9 Vrms maximum according to its datasheet. MiniDSP used to have a balanced version with a higher output, but that has been dropped. We want an amp with a gain so that 0.9 Vrms drives it to near clipping, but not over. With too much amp gain you can’t use the full DAC output (higher SNR) and with too little gain you lose available amplifier output. See the ICEpower amp analysis below.
      3. Filter stage. The overall filter gain needs to be 0 dB if we’re using the full input range. For example, let’s say we’re using a Linkwitz transform with 7 dB of gain to extend the bass roll off. That means the signal needs to be attenuated 7 dB before the DACs to avoid clipping. The ADAU1701 does have an interesting filter processing approach: after the ADCs 4 bits are added to allow an additional 24 dB of headroom during filter processing. But those 4 bits are removed just before the DAC stage. The takeaway is you have to aware of the filter gain and attenuate accordingly (see Linkwitz’s analysis: https://linkwitzlab.com/LXmini/Design.htm).

      Paying close attention to the various gain stages should minimize hiss problems and eliminate digital clipping. No guarantee to completely remove hiss (don’t forget about possible ground loops, poor power supplies, etc.), but it’s a step in the right direction. And I personally don’t put any volume control in the DSP, I always leave it in the preamp driving the DSP.

      Here’s a quick output stage analysis I did using the ICEpower amps you mentioned. The good news is the ADAU1701 won’t overdrive those amps. The flip side is you’re losing about 3 dB of available amp output.

      ICEpower 50ASX2:
      SE (stereo) version: 2 X 47W @ 1% THD+N, 20 Hz – 20 KHz, 4 Ohm load
      SE version gain = 20.5 dB (= 10.6 voltage gain). Output @ 0.9 V input = 22.75 W (4 Ohm load)
      BTL (mono) version: 1 X 170W @ 1% THD+N, 20 Hz – 20 KHz, 4 Ohm load
      BTL version gain = 27 dB (= 22.4 voltage gain). Output @ 0.9 V input = 101.5 W (4 Ohm load)

      ICEpower 50ASX2:
      SE (stereo) version: 2 X 120W @ 1% THD+N, 20 Hz – 20 KHz, 4 Ohm load
      SE version gain = 24.8 dB (= 17.4 voltage gain). Output @ 0.9 V input = 61.2 W (4 Ohm load)
      BTL (mono) version: 1 X 450W @ 1% THD+N, 20 Hz – 20 KHz, 4 Ohm load
      BTL version gain = 30.7 dB (= 34.3 voltage gain). Output @ 0.9 V input = 237.9 W (4 Ohm load)
      Last edited by ernperkins; 03-10-2022, 03:05 PM. Reason: Font
      "Everything is nothing without a high sound quality." (Sure Electronics)

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      • Scarface1
        Scarface1 commented
        Editing a comment
        even if the pre amp overdrive your amp, you can add a limiter in the DSP right?

    • #33
      Update: I've ordered and received the parts to build a 2-way electronic crossover that also allows for time-alignment of the tweeters. Features also include master gain control (can fully utilize DAC volume control), 24dB/oct Linkwitz-Riley high & low pass networks, and volume control on high pass for the tweeter. The XO2W24 PCBs are from Ampslab. This will allow me to have the following audio path: pi running Moode with CamillaDSP for BSC and any other PEQ tweaks -> DS600 DAC -> XO2W24 -> ICEpower amps -> DXT-MAX. I'll see how this compares to the ADAU1701.
      My Builds - Vigor (Nola Brio Knockoff) - Overnight Sensations w/ Voxel - Speedsters in surplus boxes - Zaph B3N's in bamboo - Classix II in BR-1 cabinets - Dual TPA3116 D2 amp in an old music box - Mariposas

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